A novel cyclopeptide from the cyclization of PACAP(1–5) with potent activity towards PAC1 attenuates STZ-induced diabetes

The N-terminal deletion of pituitary adenylate cyclase-activating polypeptide (PACAP)(1–5) generates its own antagonist. The cyclopeptide C*HSDGIC*, which results from the cyclization of PACAP(1–5) with disulfide, was designed and synthesized. CHO cells expressing a PAC1 N/R splice variant (PAC1-CHO) were used to detect the potent activation of PAC1 by C*HSDGIC*. In vitro cell assays showed that C*HSDGIC* stimulated cAMP production and increased the viability of PAC1-CHO cells at micromolar concentrations, about 1000 fold that of PACAP. PACAP(6–38) blocked the effects of PACAP on the proliferation of PAC1-CHO cells but did not interfere with the effects of C*HSDGIC*, suggesting that the activation of PAC1 by C*HSDGIC* was independent of the binding of PAC1 to the C-terminus of PACAP. In experiments in vivo, 10 μmol/kg C*HSDGIC* decreased the plasma glucose level, increased the plasma insulin level and improved glucose tolerance significantly (P < 0.01) when co-injected with STZ for 5 days. The results of these in vitro and in vivo studies of the biological characteristics of C*HSDGIC* reveal that it is a potent activator of PAC1.